A Design Procedure for Intermediate External Diaphragms on Curved Steel Trapezoidal Box Girder Bridges

Steel trapezoidal box girders are well-suited for curved bridges due to their high torsional rigidity. This bridge type features one or more steel trapezoidal girders with a cast-in-place concrete roadway slab. Once the slab is in place, the section becomes fully closed, and the stiffness and strength of the bridge cross-section significantly increases. Temporary intermediate external cross-frames, or diaphragms, are typically installed during the construction phase to limit rotations and twisting distortions in the girders in order to maintain alignment. It is important to optimize the number of intermediate external diaphragms due to their added cost for design, fabrication, installation, and removal.

A method for the design of intermediate external diaphragms is presented herein. The design procedure was developed through an investigation of torsion in curved girders and the effect of torsion on twin-trapezoidal box girder bridge systems. The design procedure was evaluated by comparing results with data from a highway bridge in Austin, Texas, that was completed in summer 2001. Two external diaphragms on the bridge were monitored during the concrete deck pours and during a live load test after the deck had hardened.